As will be familiar to those skilled in the art, in a typical ion implanter a relatively small cross-section beam of dopant ions is scanned relative to a silicon wafer. Traditionally, a batch of wafers was mechanically scanned in two directions relative to a fixed direction ion beam.
Single wafer processing has several advantages over batch processing, such as increased flexibility of implantation and a reduction in the potential costs should the implantation process fail, requiring the wafer to be discarded. Single wafer processing is particularly preferred for larger wafers having a diameter of 300 mm or more.
For single wafer processing, it is desirable mechanically to scan the silicon wafer in one direction whilst electrostatically or electromagnetically scanning or fanning the ion beam in a second direction.
U.S. Pat. Nos. 5,003,183 and 5,229,615 show examples of a variety of different scanning mechanisms which are known for this purpose. WO-A-99/13488 shows a further device suitable for allowing mechanical scanning of a single wafer. In the device of WO-A-99/13488, the wafer is mounted upon a substrate holder in a process chamber of an implantation device. Attached to, or integral with, the substrate holder is an arm which extends through an aperture in the wall of the vacuum chamber. Mechanical scanning is effected by a scanning mechanism located outside the process chamber. The scanning mechanism is connected with the arm of the substrate holder and allows movement of the arm (and hence the substrate holder) relative to the process chamber.
To facilitate movement of the moving parts of the scanning mechanism, one or more gas bearings are provided. For example, the end of the arm distal from the substrate support may be attached to a first bearing member which moves reciprocally relative to a second bearing member. This allows the wafer to be mechanically scanned in a plane orthogonal to the ion beam of the ion implanter. Movement of the first bearing member relative to the second bearing member is facilitated via a first gas bearing.
Likewise, the second bearing member may itself be rotatable relative to the process chamber to allow tilting of the substrate support relative to the direction of ion beam. The second bearing member rotates against a stator mounted upon a flange adjacent the aperture in the wall of the process chamber; a second gas bearing is employed between the stator and the surface of the second bearing member to facilitate this rotation.
For successful operation of the gas bearings, the bearing surfaces must each be flat. Variations in flatness of more than 10 .mu.m or so can cause one of the bearing surfaces to touch the other bearing surface. Whilst the bearing surface of the second bearing member and that of the stator may be made flat to this accuracy, the exterior surface of flange on the process chamber wall adjacent to the aperture therein tends to be relatively uneven. Thus, when the stator is bolted or otherwise affixed to that flange, the clamping forces generated can distort the bearing surface of the stator. This problem is exacerbated by the presence of a vacuum within the vacuum chamber: the force of atmospheric pressure on the outside (non-bearing) surface of the second bearing member can also contribute to distortion of the stator.
It is an object of the present invention to address this problem. More generally, it is an object of the invention to reduce the problems associated with distortion of the bearing faces in a fluid bearing.